Hypergolic ignition of coated composite propellant grain



Sept. l?, 1968 R s. BROWN ET AL 3,401,519

HYPERGOLIC IGNITION OF COATED COMPOSITE PROPELLANT GRAIN Filed Nov. l5, 1966 ILQZmOmA 3,401,519 HYPERGOLIC IGNITION OF COATED COMPOSITE PROPELLANT GRAIN Robert S. Brown, Santa Clara, and James E. Erickson, Sunnyvale, Calif., assiguols to the United States of America as represented by the Secretary of the Air Force Filed Nov. 15, 1966, Ser. No. 594,637 4 Claims. (Cl. 60-39.06)

ABSTRACT OF THE DISCLOSURE surface.

This invention relates to solid propellants useful for rocket propulsion. In a more specific aspect, it relates to a method rfor enhancing the hypergolic ignition characteristics of solid rocket propellants of the composite type. Composite type propellants usually comprise an intimate mixture of a finely divided oxidizer uniformly distributed in a resinous binder which acts as a fuel. Other materials may also be included as additives to improve fabrication or ballistic characteristics. For example, fillers, stabilizers, burning rate modifiers or catalysts, plasticizers, curing agents and ignition aids such as finely divided aluminum are often added to propellant formulations. The various ingredients making up the propellant composition are blended and mixed thoroughly using conventional mixing equipment. The mixture is then formed into a desired shape by compression molding, injection molding or extrusion followed by curing at either room temperature or elevated temperatures.

As the utilization of solid propellants increases, the problem of ignition becomes of paramount import-ance because of the inherent difficulty in properly igniting these materials. The ignitability of solid propellants `often varies due to condensation of moisture on the propellant surfaces and variations in propellant surfaces due to extrusion and curing phenomenon. Consequently, it often becomes difficult to ignite the propellant materials in a reproducible manner throughout a wide range of temperature condi4 tions. Also, incomplete ignition of the propellant grain generally produces relatively long periods of ignition delay or lag often resulting in misres.

Various ignition systems have been employed with some degree of success in attempting to improve the ignitability of solid propellants. One of the more successful systems, commonly referred to as hypergolic ignition, involves the use of a hypergolic reactive chemical. This technique is accomplished by spraying the surface of the propellant material with a hypergolic fluid such as chlorine trifluoride. When the fluid contacts the surface of the propellant, an exothermic reaction is initiated which raises the temperature of the propellant to its -autoignition temperature. This type of system is flexible, simple in design and can be readily employed with a variety of rocket motors. It suffers a disadvantage, however, in that unreasonably long ignition delays often occur when the hypergolic technique is employed with composite type solid propellants. This disadvantage seriously impairs the usefulness of an otherwise desirable system for igniting solid lpropellants.

Accordingly, the primary object of this invention is to provide an improved hypergolic ignition system for composite type solid rocket propellants.

nited States Patent O 3,401,519 Patented Sept. 1 7, l 968 ICC Another object of this invention is to provide a hypergolic ignition system that minimizes ignition lag or delay upon the firing of solid rocket propellants.

Still another object of this invention is to provide a hypergolic ignition system that improves the ignitability of solid rocket propellants over a wide range of temperatures, and is relatively unaffected by the surrounding chemical and moisture conditions of its environment.

A further object of this invention is to provide a hypergolic ignition system that will exhibit stability over relatively extended periods of time.

The above and still further objects, advantages and features of this invention will become readily apparent upon considering the following detailed description thereof and the accompanying 4drawing in which the figure is a graphic illustration demonstrating the advantages of this invention.

According to the invention, it has been found that the aforestated objects can be accomplished by applying silicone-base and hydrocarbon-base llubricants to the surface of composite type solid propellants. When the treated surfaces are contacted with a hypergolic reactive-type oxidizer such as chlorine trifluoride, significant decreases in ignition delays Iare -found to occur compared to ignition delay times for untreated surfaces. The ignition of the treated surfaces is also more vigorous since the ignition spreads over the entire surface of the propellant grain at extremely rapid rates.

The lubricating agents which are applied to the surfaces of the propellant grains in accordance with this invention possess extremely low vapor pressures, thereby permitting long periods of rocket motor storage without significant reduction in the original improvement in the ignition characteristics of the treated propellant grain.

These lubricants include silicone-base lubricants; such as silicone oils and silicone greases, as well as hydrocarbon lubricating oils. The hydrocarbon oils used in this invention can be derived from natural crude oil, that is mineral oils such as naphthenic oils, paraffinic oils or mixed base oils; or synthetic lubricants. The silicone lubricants used in the invention are nonresinous polymeric organosilicone compounds substantially free from reactive substituents attached to the silicon atoms. The preferred organosilicone polymers this invention is concerned with are the alkyl and aryl substituted siloxanes of varying chain lengths and room temperature viscosities from 50-1000 centistokes, such as dimethyl polysiloxane, phenylmethyl polysiloxane, diethyl polysiloxane, butylmethyl polysiloxane and trimethylsilyl end-blocked dimethyl silicone oils, as well as the above silicone oils converted to greases by conventional methods. For example, silica aerogel when combined with methyl silicone oil forms a stop-cock lubricant which can be effectively utilized with this invention as a coating agent for propellant grains.

By referring to the figure in the drawing, it can be demonstrated that the application of the lubricating agents contemplated by this invention to the surface of a propellant grain produces significant advantages in the ignition characteristics of the treated propellant. The test equipment used in obtaining the results outlined in the graph of the figure was a box or chamber with windows on two sides. Propellant samples 1.75 inches in diameter, both treated and untreated, were placed on a canted pedestal which was located at the center of the box floor. The combustion products were exhausted through a vent in the side of the combustion chamber. The hypergolic reactive fluid was injected through an injector and a solenoid valve assembly located in the top of the chamber and onto the propellant surface. By control of the injector pressure drop and the duration of the current flow through 3 the solenoid, the rate of hypergol flow to thepropellant surface and the duration of the iiow can be controlled within predetermined parameters. Ignition of the propellant grain `was detected by a photocell which viewed the propellant surface through one of the windows on the side of the chamber. Thus, by simultaneously recording the current through the solenoid and the photocell output on an oscillograph recorder, the total system ignition delay (apparatus delay time plus intrinsic delay) could be determined from the time difference between the two signals. The apparatus delay time was determined separately so that the true intrinsic ignition delay (from first hypergol-propellant contact to ignition) could be determined.

were cured in open planchettes at 160 F. as in Example 1. After curing the surface was spray-coated with a thin coat of a silicone oil lubricant. (Sprayon No. 205 from Sprayon Products, Inc., a silicone-base mold release agent was the specific lubricant utilized.)

EXAMPLE 4 A fourth series of samples of the propellant of Table I were cured in a manner similar to that of Example 1 except the free surface of the cured propellant was coated with a thin layer of a hydrocarbon base lubricating oil.

EXAMPLE 5 A iifth series of samples of the propellant of Table I were cured in accordance with the procedure outlined This invention is applicable to any of the composite type solid propellants, but has been found especially l Example 1 above efeept the free Surface Qf the PFO- useful with solid propellants of the aluminized amimo- Penal1t Was Covered Wlth a thin `layer of a silicon base nium periiorate type having a polybutadiene-acrylic acid- Vacuum grease (DOW comme H1 V0e Grease)' acrylonitrile binding system. Table I sets forth a specic The resalte Produced from testing the'untfeated Pfeexample of this type of solid propellant and represents Peuant grains 0f EXamPleS 1 and 2 and me tfeate PTO the propellant material used in the examples of this Pellant grams 0f Examples 31 4 and 5 are ShOWU 1U the invention TABL I graph of the figure and also in Table II.

4 E Percent by TABLE H sUMMARY 0F IGNITION DATA FREE SURFACE CAST 1N AIR Composition: weight 2r P u t A P 1 s t I t.

v1`0 0 an V fa e ll Se YS GID n TlIlSlC Ammmum perchlorate 68 o Run No. Batlh No. Pressue, Duration, Delay Delay Aluminum 16 p.s.i.a. msec. Bmdfi? e- 16 65 23 10o 85 95 63 Additives 0 66 23 190 S5 85 53 67. 23 100 120 so 5s A propellant grain was prepared in accordance with 23 100 75 50 2s the formulation of Table I and compounded in a conven- 11%?) 2g gg gg tional manner. The surface of the propellant grain was 23 100 35 o@ treated by coating with either a silicone or hydrocarbon 2g gg gg gg lubricant. The propellant was then placed in the test 4 110 75 55 33 chamber described above in order to obtain the results 83 13g gg set forth in the graph of the figure. The following ex- 4 100 so 50 28 amples illustrate specific embodiments of the invention. gg g3 gg gg 4 100 73 45 23 EXAMPLE 1 34 50 so sg 5s 34 50 65 15 128 The propellant of Table I was free cast in air and 34 50 105 65 43 cured in open planchettes at 160 F. The free surface gg lg gg gg being the surface which would eventually be contacted 34 5o 10o 80 58 by the hypergolic fluid used to initiate burning of the gg 75g g 12g propellant grain. 20g g5 133 9 0 28 EXAMPLE 2 34 190 12o 65 43 A second sample of the solid. propellant of Table I gi 12g 391g sgg was cured at 160 F. on an aluminum surface which had 40 190 55 12o 98 been previously coated with a silicon rubber type mold ig E?, 2g gg ig release agent, commonly referred to as RTV. 193 6o so 5s 4o 195 so 60 38 EXAMPLE 3 40 195 95 so 5s A third series of samples of the propellant of Table I l Initial fire may not have been on propellant surface.

FREE CAST MODIFIED SURFACE STUDIES Run No. Propellant Average Pulse System Intrinsic Surface Conditions Batch Pressure Duration Delay Delay 40 198 85 30 8 Applied Sprayon No. 205. Mold Release just below test.

40 05 95 30 8 Applied Lubricating Oil.

40 95 85 25 3 Applied Sprayon No. 205 to M of surface.

40 95 35 25 3 Applied Lubricating Oil.

95 95 40 18 Applied Sprayoii No. 205.

45 95 110 35 13 Applied Dow Corning Hi Voc Grease.

MOLD CAST SURFACE Run No Propellant Average Pulse System Intrinsic Comments Batch Pressure Duration Delay Delay 23 100 120 150 128 Cast ori Aluminum without mold. 45 100 115 40 18 Cast; on Aluminim with release. 45 100 120 180 158 Cast on Aluminum without release. 57 100 65 170 148 Cast on RTV coated Aluminum. 57 100 75 65 43 Do. 57 100 55 95 73 Do. 57 100 100 80 58 D0. 57 100 155 65 43 D0. 57 100 115 75 53 Do. 57 100 85 65 443 Do.

The data set forth in Table II and graphically illustrated in the drawing demonstrates that the first two types of surface have equivalent ignition characteristics and have a minimum ignition delay with the chlorine triuoride hypergol of 55 milliseconds or greater. However, the propellant grains of Example 3, 4 and 5, the surfaces of which were treated in accordance with this invention, are shown to have ignition delays of less than 20` milliseconds with the same hypergol and same hypergolic flow rates. These results demonstrate that the treatment of the surfaces of compositive type solid propellant grains with the lubricating agents 0f this invention significantly enhances the hypergolic ignition characteristics of the propellant grain. The ignition delay is considerably reduced and the rate of ignition propagation to the entire propellant surface is greatly increased.

While the invention has been described with particular reference to specific embodiments thereof, it is to be clearly understood that the present disclosure of the specific embodiments has been made only by way of illustration and that numerous changes in the details of this invention may be resorted to without departing from the spirit and scope of the invention as hereinafter claimed.

What is claimed is:

1. A method for uniformly igniting a composite type solid propellant grain which comprises the steps of applying a coating to the ignition surface of a composite type solid propellant grain, said coating comprising an ignition enhancing agent selected from the group consisting of hydrocarbon-base oil lubricants, silicone-base oil lubricants and silicone-base grease lubricants, contacting said surface coated grain with a hypergolic fluid capable of penetrating to the ignition surface of the said grain in order to initiate an exothermic reaction and raise the temperature of the said grain to its autoignition temperature,

2. A method in accordance with claim 1, wherein said lubricating agent is a silicone-base oil.

3. A method in accordance with claim 1, wherein said lubricating agent is a silicone-base grease.

4. A method in accordance with claim 1, wherein said lubricating agent is a hydrocarbon-base petroleum oil.

References Cited UNITED STATES PATENTS 3,120,184 2/1964 Ellern 10Q-37.8 3,147,710 9/ 1964 Gluckstein 102-70 3,250,829 5/1966 Wall l49-2 X BENJAMIN R. PADGETT, Primary Examiner. 

